A thermistor's electrical resistance changes as a function of its temperature, which is the basic principle that enables thermistors to be used for sensing fluid temperatures or liquid levels.
For example, to sense whether a liquid level has reached a certain upper limit, a thermistor with a positive or negative temperature coefficient can be energized to electrically heat the thermistor to a temperature above that of the liquid. Then, if the liquid level is below the thermistor, the relatively low heat transfer rate between the electrically heated thermistor and the gas above the liquid allows the thermistor's temperature to remain elevated. If, however, the liquid level rises to that of the thermistor, the cooler liquid quenches the thermistor, thereby changing the thermistor's electrical resistance. The thermistor's electrical resistance can thus be monitored as a means for determining whether the liquid level is above or below the thermistor. An example of a previous use of a thermistor to measure temperature in a refrigerant line is shown in commonly assigned U.S. Pat. No. 4,987,749 to Baier, which is hereby incorporated by reference.
When a thermistor is used as a temperature sensor to determine the actual temperature of a fluid, it is not necessary to electrically heat the thermistor. Instead, the varying temperature of the fluid itself is what changes the thermistor's temperature and thus changes its electrical resistance as well. Unfortunately, however, the thermistor's temperature lags a fluid's changing temperature due to a limited heat transfer rate between the thermistor and the surrounding fluid.
The thermistor's delay in reaching the temperature of the surrounding fluid is not always a problem, but it can be in certain applications. When this technology, for example, is used in a conventional manner to sense the oil/refrigerant fluid conditions within the sump of a refrigerant compressor, the fluid conditions can change so suddenly that the thermistor's electrical resistance might inaccurately represent the actual conditions within the sump. As a result, the thermistors might fail to detect a fluid related problem.
At startup, for instance, a refrigerant compressor might experience a rapid loss of oil due to excessive foaming within the compressor's sump. Such foaming can be caused by a suction line blockage, closed expansion valve, closed service valve, or some other problem. If the problem causes the suction pressure to fall quickly and low enough, the refrigerant mixed in the sump oil will flash, which can suddenly produce an expanded foamy mixture of oil and refrigerant vapor. If a thermistor is too slow to detect the rapid change in fluid level or temperature, the control system might allow the compressor to continue operating under these conditions. Thus, the compressor might ingest the foamy mixture, creating a liquid slugging problem, and discharge the mixture, thereby losing oil that is needed for ongoing compressor operation.
Although various types of sensors might be used for detecting such problems, conventional sensors with existing control schemes can be too slow to react in time to protect the compressor, and faster control schemes can be too expensive. Consequently, there is a need for a cost effective way of accurately detecting and responding to sudden adverse conditions of a refrigerant compressor.